class Solution
{
public:
    int singleNonDuplicate(vector<int> &nums)
    {
        int left = 0;
        int right = nums.size() - 1;
        while (left < right)
        {
            int mid = (left + right) / 2;
            if (mid & 1)
            {
                if (nums[mid] == nums[mid - 1])
                {
                    left = mid + 1;
                }
                else
                {
                    right = mid;
                }
            }
            else
            {
                if (nums[mid] == nums[mid + 1])
                {
                    left = mid + 1;
                }
                else
                {
                    right = mid;
                }
            }
        }
        return nums[left];
    }
};